Research on optical imaging apparatuses which emit light from a light source such as a laser into a living body and which create images that represent information about the inside of the living body which is obtained on the basis of the incident light has been actively carried out in the medical field. One technique in this optical imaging technology is photoacoustic imaging (PAI). In PAI, a living body is irradiated with pulsed light produced from a light source, acoustic waves (typically, ultrasonic waves) are detected which are produced from the tissue of the living body that absorbs the energy of the pulsed light which propagates and diffuses in the living body, and imaging (creation of an image) is performed on the basis of the detected signals so that the created image represents live body information.
That is, by utilizing a difference between the light energy absorption ratio of a specimen area such as a tumor and that of other tissue of the specimen, elastic waves which are produced when the specimen area absorbs received light energy and momentarily expands are detected by an acoustic wave probe (also referred to as a probe or a transducer) which is a detector. The detected signals are analyzed so that an image that represents an optical characteristic value distribution, such as an initial pressure distribution, an optical energy absorption density distribution, or an optical absorption coefficient distribution, is obtained.
These pieces of image information can be utilized for quantitative measurement of a specific substance in a specimen, such as hemoglobin concentration included in blood, or oxygen saturation of blood, by measuring the specimen by means of light having various wavelengths.
Recently, by using this photoacoustic imaging, preclinical research on imaging of a vessel image of a small animal, and clinical research on application of this principle to diagnosis of, for example, breast cancer, prostatic cancer, and carotid plaque have been actively carried out.
In photoacoustic imaging, an initial sound pressure (P0) for photoacoustic waves produced from an absorber due to light absorption may be expressed by the following equation.[Math. 1]P0=Γ·μa·Φ  (1)
Here, Γ represents a Grueneisen coefficient, which is obtained by dividing the product of a thermal expansion coefficient (β) and the square of the speed of sound (c) by a specific heat at constant pressure (CP). The symbol μa represents the optical absorption coefficient of an absorber, and Φ represents a quantity of light (local optical fluence) with which the absorber is irradiated. The Grueneisen coefficient Γ is known to have a substantially constant value for the same tissue. Accordingly, by measuring and analyzing changes in a sound pressure P, which is the magnitude of an acoustic wave, in multiple positions, the product of μa and Φ, that is, an optical energy absorption density distribution can be obtained.
In photoacoustic imaging, as understandable from Equation (1) described above, to determine an absorption coefficient (μa) distribution in a specimen, which is required for quantitative imaging, from the measurement result of a sound pressure (P), it is necessary to determine a quantity of light (Φ), with which an absorber that produces photoacoustic waves is irradiated, in some way. However, light introduced into the inside of a specimen (especially, a living body) exhibits behavior such as strong diffusion, so that it is difficult to estimate a quantity of light with which an absorber is irradiated.
Therefore, when no specific measures are taken, photoacoustic imaging has a problem in that only an image that represents an optical energy absorption density (μa×Φ) distribution or an initial sound pressure (P0) distribution obtained by multiplying the optical energy absorption density distribution by Γ can be generated on the basis of the measurement results of sound pressures of acoustic waves. For example, even when light absorbers are composed of the same substance and have the same absorption coefficient, such light absorbers are displayed in different contrasts in accordance with an influence of a local fluence distribution in the living body (for example, a distance from a light source).